Fuel cells comprising an electrolyte membrane, an oxidant electrode provided on one side of the electrolyte membrane, and a fuel electrode provided on the other side of the electrolyte membrane, wherein electric power is generated from an oxidant gas such as air containing oxygen that is supplied to the oxidant electrode, and a fuel gas containing hydrogen that is supplied to the fuel electrode are already known (for example, see JP H05-47394 A, JP 2004-127914 A, and JP 2004-220794 A).
JP H05-47394 A discloses technology for controlling the water content state of a solid polymer electrolyte of a fuel cell in an optimal state, thereby enabling stable operation of the cell. Specifically, a fuel cell is disclosed in which when the current-voltage characteristics of the fuel cell deteriorate and the conductivity of the solid polymer electrolyte increases, the amount of moisture contained within the supplied reactant gas is reduced, whereas when the current-voltage characteristics of the fuel cell deteriorate and the conductivity of the solid polymer electrolyte is low, the amount of moisture within the reactant gas is increased.
Further, JP 2004-127914 A discloses technology for controlling the pressure difference between the pressure of the oxidant gas and the pressure of the fuel gas in accordance with the result of diagnosing the state of moisture within the fuel cell. Specifically, in JP2004-127914A, when the moisture content is diagnosed as being excessive, the pressure difference is controlled so that the pressure of the fuel gas is increased relative to the pressure of the oxidant gas, and as a result, diffusion of water from the air electrode side to the fuel electrode side is inhibited, thus preventing water accumulation within the electrode region of the fuel electrode. On the other hand, when the moisture content is diagnosed as being insufficient, the pressure difference is controlled so that the pressure of the fuel gas is lowered relative to the pressure of the oxidant gas, and as a result, migration of water from the air electrode side to the fuel electrode side is promoted, enabling water impregnation of the electrolyte membrane to occur rapidly.
Furthermore, JP 2004-220794 A discloses a technology for maintaining the pressure difference between the anode pressure and the cathode pressure at a constant level without the use of an anode pressure sensor or cathode pressure sensor, by using the change in the current-voltage characteristics of the fuel cell caused by excessive or insufficient supply volume or pressure of the hydrogen gas or air. Specifically, JP 2004-220794 A discloses a technique wherein when the actual voltage value is lower than a previously stored standard voltage value that is based on the current-voltage characteristics of the cell, this reduced voltage is corrected by increasing the pressure of the fuel gas or the oxidant gas. This technique utilizes the fact that a judgment as to whether or not the supply of hydrogen or air is insufficient can be made by comparing the actual voltage value with the standard voltage value.